Recently, a lot of effort and energy is put into designing next generation optical access (NGOA) networks that can alleviate the data rate bottleneck that is present in current access networks. Many different next generation optical access (NGOA) networks architectures have been proposed. A very promising architecture has been recently been proposed in H. Rohde “Next Generation Optical Access: 1 Gbit/s for everyone”, invited talk at ECOC 2009.
Coherent detection is a key factor to enable the key parameters: 1 Gbit/s sustained symmetrical data rate per user, up to 100 km reach and a splitting factor of up to 1000.
FIG. 1 is a schematic representation of a high level architecture of a next generation optical access (NGOA) network.
FIG. 2 is a schematic representation of possible DAC based realization of an OLT transmitter and the optical spectrum at the output.
The data that is to be sent is first pre-processed by a DSP unit and digitally up-converted to intermediate frequencies. Subsequently the real and imaginary part are separated and fed to two DACs.
The outputs of the DACs are then used to modulate the complex signal on an optical carrier using an IQ modulator. On the right side of FIG. 2 the optical spectrum of the transmitter output is shown. In this illustration 3 subcarriers are shown.
In between the subcarriers a guardband is present in order to allow the signal from the ONU to be sent back to the OLT on the same fiber without being impaired by back-reflections. The spectral guardband that is required is at least as wide as the bandwidth of the subcarriers itself and this means that for the DACs less then half of the bandwidth can be used effectively for the generation of subcarriers.
The main disadvantage of this implementation is that the guardband required in between the subcarriers reduces the efficiency of the DAC (the number of subcarriers that can be generated with a single OLT).
In conventional DAC based realization of OLT transmitters, DAC are used in association with guardbands between the subcarriers, which may result in an inefficient use of the DACs
Alternatively analogue electrical IQ mixers could be used to up-convert all individual subcarriers. However, these analogue components are relatively expensive.
The problem to be solved is to overcome the disadvantages stated above and in particular to provide a solution which allows increasing the efficiency of an OLT transmitter in a cost effective way.